1. Field of the Invention
The present invention relates to input/output (I/O) that is implemented by an uppermost computer environment in environments having virtualization mechanisms overlaid at plural stages.
2. Description of the Related Art
One example of the related art in the field of the present invention is disclosed in JP-2009-003749-A. This publication discloses that “by selecting either one of a guest status area 221 for executing a user program on a second virtual processor in accordance with a factor of calling a host virtual machine manager (VMM) and a host status area 222 for executing a guest VMM, and updating a guest status area 131 for controlling a physical processor in a shadow virtual machine control block (VMCB), a next generation operating system (OS) having a virtualization function is executed as the user program on a first virtual processor”.
JP-2009-003749-A describes a means that causes an OS having a virtualization mechanism to operate, on a virtual computer formed with a virtualization technology, on a physical computer, or so-called “multistage virtualization technology”. By using multistage virtualization technology, an environment that includes virtual computers at plural stages can be constructed.
The multistage virtual computer environment has an advantage in that a flexible system configuration can be set up. Conversely, the multistage virtual computer environment has a problem in that deterioration in performance is likely to occur because in order for a virtual computer at an uppermost stage to access a virtual computer at a lowermost stage, the accessing needs to be established via a virtual computer and virtualization mechanism at an intermediate stage.
For example, it is assumed that in a two-staged virtual computer environment, an I/O request is issued from an OS on a virtual computer at the second stage to a physical device. The I/O request executed on the virtual computer at the second stage is received by an OS on a virtual computer at the first stage. The OS on the virtual computer at the first stage executes the received I/O request. This I/O request is received by a virtualization mechanism on a physical computer. The virtualization mechanism issues an I/O request to the physical device.
This example indicates that “the I/O request executed on the virtual computer at the second stage is executed via two layers of the virtualization mechanism on the physical computer”. This indicates that performance of the I/O request executed by the OS on the virtual computer at the second stage deteriorates than performance of the I/O request executed by the OS on the physical computer and the virtual computer at the first stage.